Portrait of Einstein : EINSTEIN'S LEGACY

by Robert, L., Forward

portions of article published in OMNI magazine, March 1979, p.54.
"All it took to invent the laser was some understanding of Einstein's ideas on stimulated emission of radiation."

Some theorists were on the right track, especially Planck, who proposed that nature acted by using "quanta" of energy. But it was the young, unknown Albert Einstein who explained everything and started the field of quantum mechanics with his paper on the photoelectric effect. Einstein showed that light does not consist of continuous waves, nor of small, hard particles. Instead, it exists as bundles of wave energy called photons. Each photon has an energy that corresponds to the frequency of the waves in the bundle. The higher the frequency (the bluer the color), the greater the energy carried by that bundle.

Indeed, Einstein's Nobel Prize was not awarded for either one of his relativity theories - the Nobel Commitee thought them too speculative at the time. Rather Einstein won the prize for explaining the photoelectric effect. Two of Einstein's 1905 papers were on the theory of atoms and molecules, yet there were still many scientists in 1905 who did not believe in atoms or molecules !

Image of S.N.Bose : In 1924 Einstein received a paper from Indian physicist S.N.Bose that described light as a gas consisting of photons. This photon gas was a strange type of gas, for the particles in it did not obey the commonsense statistical laws that billiard balls do. If you randomly roll a number of perfectly elastic billiard balls on a frictionless table, sooner or later they will all end up in one pocket or another. If you checked by repeated experiments, you would find that all the balls had an equal probability of falling into any one of the pockets. But if the billiard balls behaved like photons, you would find that if one of the pockets already had a ball in it, the rest of the balls would have a tendency to fall into that pocket. In fact, the more balls already in a pocket, the more likely another ball would choose to join its identical mates.

Semi-Classical Statistics

Particle distribiution Two distinguishable particles, the red and blue balls, can occupy three states in nine different ways. There are three chances in nine to obtain particles in the same state.

Photon Statistics

Boson distribiution If these particles were indistinguishable like photons, there would be fewer possibilities; three less than the previous case. This enhances the probability (3 chances in 6) of obtaining photons in the same state. Now, there is no force or attraction involved in this effect. It is just a statistical tendency that causes photons to prefer to travel together. This phenomenon, developed and mathematically expressed by Bose and Einstein is what makes a laser !

Although Einstein did not invent the laser his work laid the foundation. It was Einstein who pointed out that stimulated emission of radiation could occur. He used his photon mathematics to examine the case of a large collection of atoms full of excess energy and ready to emit a photon at some random time in a random direction. If a stray photon passes by, then the atoms are stimulated by its presence to emit their photons early. More remarkably, the emitted photons go in the same direction and have exactly the same frequency as the original photon ! Later, as the small crowd of identical photons moves through the rest of the atoms, more and more photons will leave their atoms early to join in the subatomic parade.

All it took to invent the laser was for someone to find the right kind of atoms and to add reflecting mirrors to help the stimulated emission along. Remember, the acronym LASER means Light Amplification by (using Einstein's ideas about) Stimulated Emission of Radiation.

(reprinted by permission of OMNI Publications International, Ltd. )

"A splendid light has dawned on me..."

- Albert Einstein

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